In recent years, multi-layer transformers have attracted attention as multi-layer magnetic parts that are thin, small, and lightweight in accordance with rapid advances in the miniaturization of electronic devices. FIG. 6 is a disassembled perspective view of a stacked body of a conventional multi-layer transformer. FIG. 7 is a vertical cross-sectional view along the line VII-VII in FIG. 6 after stacking. The description below is based on FIGS. 6 and 7.
A conventional multi-layer transformer 80 comprises primary-winding magnetic sheets 82b and 82d on which primary windings 81a and 81c are formed, secondary-winding magnetic sheets 82c and 82e on which secondary windings 81b and 81d are formed, and magnetic sheets 82a and 82g that hold the magnetic sheets 82b to 82e from both sides.
Furthermore, a magnetic sheet 82f for improving the magnetic saturation characteristic is inserted between the magnetic sheet 82e and magnetic sheet 82g. The magnetic sheets 82a to 82e are provided with through-holes 90, 91, and 92 that connect the primary windings 81a and 81c and through-holes 93, 94, and 95 that connect the secondary windings 81b and 81d. The lower face of the magnetic sheet 82a is provided with primary-winding external electrodes 96 and 97 and secondary-winding external electrodes 98 and 99. The through-holes 90 to 96 are filled with a conductor. The magnetic sheets 82a to 82g are the core of the multi-layer transformer 80.
Further, FIGS. 6 and 7 are schematic diagrams and, therefore, strictly speaking, the number of windings of the primary windings 81a and 81c and secondary windings 81b and 81d and the positions of the through-holes 90 to 96 do not correspond in FIGS. 6 and 7.
On the primary side of the multi-layer transformer 80, the current flows in the order of the external electrode 96, through-hole 92, primary winding 81c, through-hole 91, primary winding 81a, through-hole 90, and then the external electrode 97 or in the reverse order. On the other hand, on the secondary side of the multi-layer transformer 80, the current flows in the order of the external electrode 99, the through-hole 95, the secondary winding 81d, the through-hole 94, the secondary winding 81b, the through-hole 93, and then the external electrode 98 or in the reverse order. The current flowing through the primary windings 81a and 81c produces a magnetic flux 100 (FIG. 7) in the magnetic sheets 82a to 82g. The magnetic flux 100 produces an electromotive force corresponding with the winding ratio in the secondary windings 81b and 81d. The multi-layer transformer 80 operates thus.
Here, supposing that the self-inductance of the primary windings 81a and 81c is L1, the self-inductance of the secondary windings 81b and 81d is L2, the mutual inductance of the primary windings 81a and 81c and the secondary windings 81b and 81d is M, and a magnetic coupling coefficient k is defined by the following equation:k=|M|/√{square root over ( )}(L1·L2)(k≦1)
The magnetic coupling coefficient k is one of the indicators of the transformer function and the larger the magnetic coupling coefficient k, the smaller the leakage magnetic flux (leakage inductance) becomes and, therefore, the power conversion efficiency is high.
In the multi-layer transformer 80, because there is a magnetic body layer (magnetic sheets 82c to 82e) between the primary windings 81a and 81c and the secondary windings 81b and 81d, a leakage magnetic flux 101 (FIG. 7) is produced and, therefore, an adequate magnetic coupling coefficient k is not obtained. In order to resolve this problem, a technology (referred to as the ‘prior art’ below) that provides a dielectric layer (not shown) on the primary windings 81a and 81c and secondary windings 81b and 81d by means of screen printing or the application of paste and reduces the magnetic permeability of the magnetic body layer by means of a material that provides diffusion from the dielectric layer may be considered.
Problem to be Solved
However, the prior art is confronted by the following problems.
As a result of the diffusion of a conductive material (Ag particles, for example) from the primary windings 81a and 81c and secondary windings 81b and 81d to the conductor paste applied to the primary windings 81a and 81c and secondary windings 81b and 81d, there has been the risk of a reduction in the insulation of the primary windings 81a, primary windings 81c, secondary windings 81b and secondary windings 81d. The paste is in liquid form as a result of an organic solvent or the like, for example, and, therefore, the material is readily dispersed.
Further, even when the leakage magnetic flux is reduced by providing a dielectric layer, the gap between the primary windings 81a and 81c and secondary windings 81b and 81d widens to become ‘magnetic body layer+dielectric layer’. This means that the leakage magnetic flux readily enters the gap and, therefore, acts conversely in the direction in which the magnetic coupling coefficient k is reduced. Therefore, with the prior art, it is very difficult to increase the magnetic coupling coefficient k.